地下空间逾渗与裂缝属性的关系分析

裂缝连通性评价是地下空间研究的重要内容。逾渗分析是研究裂缝网络连通性的一种有效手段。裂缝网络逾渗临界值多使用裂缝间接表征参数(如分形维数)来确定,但存在裂缝网络连通性不同却有相同间接表征参数的情况,这降低了预测的可靠性。为避免此类问题并更加准确快速地表征裂缝网络的连通性,文章在构建逾渗临界值方程时,摒弃了间接参数简化的方式,使用裂缝直接表征参数(如裂缝数量、长度等)进行非线性拟合。通过二维数值模拟建立不同参数的离散裂缝网络模型,分析逾渗低、中、高概率临界条件与裂缝直接表征参数的关系,建立逾渗临界条件预测方程,并对方程在不同尺度研究区的应用进行了讨论和验证。结果表明：预测方程可有效地预测不同概率的逾渗临界值。同时文章在预测方程的基础上,建立了裂缝网络连通性评价标准,这对于裂缝发育地区的地下空间评价工作具有一定的指导和借鉴意义。     

A Connectivity Index for Discrete Fracture Networks

Connectivity is an important measure for assessing flow transport in rock, especially through fractures. In this paper, rock fracture systems are modelled by a discrete fracture model simulated by a marked point process. A connectivity index is then introduced to quantify the connectivity between any two points in space. Monte Carlo simulation is used to evaluate the connectivity index for stationary cases and relationships between the connectivity index and the parameters of the discrete fracture model are analysed. The average number of intersections per fracture, X_f, and the fracture intensity, P12 (P32), are calculated and the relationships between these parameters and the connectivity index are investigated, concluding that X_f is the more suitable parameter for the classification of rock mass flow properties. The relationships between the connectivity index and the percolation state of the fractured medium are also discussed. An edge correction is briefly discussed and a practical example is used to demonstrate the method of computing the connectivity index.     

Relationship Between the Geometric Parameters of Rock Fractures,the Size of Percolation Clusters and REV

Modeling fractured rocks with numerical methods requires some derived parameters, among which the fracture network connectivity and the size of the representative elementary volume (REV) are both of crucial importance. Percolation and REV analyses were made by the RepSim code. The program uses input parameters such as fractal dimension of the fracture midpoints (D _c), length exponent (E) and relative dip (α ^r) data. For percolation analysis, the relative sizes of the largest percolation clusters have been calculated by stochastic realizations of the simulated fracture networks with different parameter triplets. Furthermore, fracture networks can be classified into three major types on the basis of their (E,D _c,α ^r) parameters. For the REV calculations, the porosity of the generated fracture network was calculated. The derived REV size of a fracture network depends essentially on input parameters and shows a decreasing tendency with increasing D and E and vice versa. The method mentioned above was tested on both metamorphic samples of the Pannonian Basin and Variscan granitoid rocks of the Mórágy Complex. Percolation values predicted for the Mórágy granite are highly sensitive to alterations in the input parameters. The amphibolite bodies displayed a modeled fracture network with 80 to 90% of all fractures being interconnected, while the largest achievable percolation cluster size of gneiss is less than 10%. The REV size of the amphibolite is about 20 m as a result of connected fractures filling the whole body, while gneiss has lower porosity and higher REV (approximately 70 m).     

The role of in situ stress in determining hydraulic connectivity in a fractured rock aquifer (Australia)

Fracture network connectivity is a spatially variable property that is difficult to quantify from standard hydrogeological datasets. This critical property is related to the distributions of fracture density, orientation, dimensions, intersections, apertures and roughness. These features that determine the inherent connectivity of a fracture network can be modified by secondary processes including weathering, uplift and unloading and other mechanisms that lead to fracture deformation in response to in situ stress. This study focussed on a fractured rock aquifer in the Clare Valley, South Australia, and found that fracture network connectivity could be discriminated from several geological, geophysical and hydrogeological field datasets at various scales including single well and local- to regional-scale data. Representative hydromechanical models of the field site were not only consistent with field observations but also highlighted the strong influence of in situ stress in determining the distribution of fracture hydraulic apertures and the formation of hydraulic chokes that impede fluid flow. The results of this multi-disciplinary investigation support the notion that the hydraulic conductivity of a fracture network is limited to the least hydraulically conductive interconnected fractures, which imposes a physical limit on the bulk hydraulic conductivity of a fractured rock aquifer.     

孔隙裂隙双重介质的三维逾渗数值模拟研究

基于三维孔隙介质的逾渗模型,首次把裂隙这一重要的渗透通道引入到三维逾渗研究中,提出了孔隙裂隙三维逾渗的研究方法,并建立了孔隙裂隙双重介质三维逾渗模型,这一模型的建立使得逾渗理论的研究成果可以被应用到更多的领域中,如煤体、岩体等。基于VC++6.0开发了孔隙裂隙双重介质三维逾渗模拟软件,模拟研究了双重介质的逾渗规律,模拟研究表明：裂隙的存在在很大程度上提高了介质的逾渗概率,使孔隙裂隙双重介质的逾渗规律明显不同于孔隙介质;随孔隙率、裂隙分形维数、裂隙数量分布初值由小到大逐渐增长,必然发生逾渗转变的自然现象。     

Percolation in the Crust

Considering the Earth's Crust as a heterogeneous medium, we show here that the general features of the seismic, electrical, and mechanical properties of the Crust can be conveniently described with a unified model derived from percolation theory. The basic assumption is that fracture density is a variable parameter which decreases with depth and as a result there exists a critical threshold below which fractures are only connected within finite clusters. This threshold corresponds to a critical fracture density and to a critical depth. Fluids circulate through a connected network of fractures down to the critical depth Z_c. Below Z_c, anisotropic fracture clusters result in seismic reflectors and high electrical conductivity. Earthquakes are restricted to depths shallower than Z_c, where friction controls the mechanical resistance to fluid infiltrated pre-existing fractures.     

Review: The state-of-art of sparse channel models and their applicability to performance assessment of radioactive waste repositories in fractured crystalline formations

Laboratory and field experiments done on fractured rock show that flow and solute transport often occur along flow channels. ‘Sparse channels’ refers to the case where these channels are characterised by flow in long flow paths separated from each other by large spacings relative to the size of flow domain. A literature study is presented that brings together information useful to assess whether a sparse-channel network concept is an appropriate representation of the flow system in tight fractured rock of low transmissivity, such as that around a nuclear waste repository in deep crystalline rocks. A number of observations are made in this review. First, conventional fracture network models may lead to inaccurate results for flow and solute transport in tight fractured rocks. Secondly, a flow dimension of 1, as determined by the analysis of pressure data in well testing, may be indicative of channelised flow, but such interpretation is not unique or definitive. Thirdly, in sparse channels, the percolation may be more influenced by the fracture shape than the fracture size and orientation but further studies are needed. Fourthly, the migration of radionuclides from a waste canister in a repository to the biosphere may be strongly influenced by the type of model used (e.g. discrete fracture network, channel model). Fifthly, the determination of appropriateness of representing an in situ flow system by a sparse-channel network model needs parameters usually neglected in site characterisation, such as the density of channels or fracture intersections.     

Fracture analysis in Tabnak hydrocarbon field of Iran by using fractal geometry and multi-fractal analysis

High gas production from the Dashtak formation of Tabnak hydrocarbon field in Fars province, Iran, indicates the presence of natural fractured reservoir whose production potential is dominated by the structural fracture. The connectivity of fractured media depends upon the power?Claw exponent and the fracture density. Fracture pattern traces obtained from the outcrops of producing formations of six different stations in Tabnak hydrocarbon field. 2D fracture network maps of Tabnak hydrocarbon field have been analyzed from their scaling properties. The fractal analysis of fracture intensity showed heterogeneous multi-fractal structure characterized by generalized dimensions. Distribution of fracture lengths exhibits power?Claw behavior with specific exponent. Scaling laws serve to make extrapolations and to study the fracture connectivity related to scale. Fracture distribution model and reservoir productivity can be estimated, which are of great interest in decision making to optimize gas production.     

Complexity of seismic process; measuring and applications — A review

Recent methods of analysis of so called disordered systems show that many objects and processes that earlier were considered as completely random reveal clear evidence of having some ordered structure in both time and space. These new methods (fractals, percolation, nonlinear dynamics and complexity theories) allow visualization and quantitative assessment of the level of complexity (orderliness) of these structures, using both theoretical models and experimental data. We consider sequentially some aspects of structural and evolutionary complexity of dynamics of seismic process and the technique of measuring this property.It is shown that the physical properties of geophysical medium are not always self-consistent and manifest fractal behavior on selected spatial and temporal scales. Mechanical percolation theory can be used for modeling geometry of fracture process. Namely, we consider fractal and connectivity aspects of delayed failure, including energy emission during fracturing. Special attention is paid to relating the intensity of geophysical anomalies to the strain in the framework of the pressure-induced anomalous strain-sensitivity (percolation) model, which explains naturally the observed heterogeneity of response of a geophysical media to the strain variation.Different methods of measuring the dynamic complexity of seismological time series are applied to magnitude and waiting time sequences of Caucasian earthquakes. The fractal (correlation) dimension d₂ of the latter is high (larger than 8), but the former one has as low dimension as 1.6–2.5, which makes waiting time sequences a promising tool for revealing precursory changes.The same nonlinear technique allow detecting significant changes in the seismic regime during external electromagnetic forcing by MHD pulses; similar tests on the laboratory scale show the possibility of triggering/controlling stick-slip process by relatively weak electromagnetic or mechanical forcing.Lastly, the predictive potential of complexity analysis of seismological time series is considered. For example, percolation model predicts the increase of the number of large events and the scatter of magnitudes of events, decrease of the magnitude-frequency relation slope and appearance of multifractality at approaching the final rupture.It seems that seismology can benefit from using the new techniques to cope with the complexity of earthquake machine; for example, the measures of complexity can be characteristic for a given region and change before strong earthquake.     

三维网络结构岩体力学架构与展望

三维网络结构岩体力学是岩体力学的一个分支。根据笔者20余年对裂隙岩体研究的心得,阐述了三维网络结构岩体力学的主要特点、研究对象,基本架构、主要研究内容与未来的展望。将岩体结构控制论归结为材料强度控制、结构类型控制、渗流通道控制和几何边界控制4个方面。技术层面的主要内容包括三维RQD估算、岩体表征单元估算技术、结构块体搜索技术、定向投影岩体结构面三维连通率估算技术以及岩体三维渗径搜索技术等。理论层面主要由球面投影理论、概率统计理论、随机动力学、几何拓扑理论、块体理论、非线性系统理论和裂隙岩体多尺度等效结构模型等构成。强调足量的随机不连续面地质信息的原位统计取样是三维网络结构岩体力学的基础,岩体随机不连续面三维网络数值模拟技术是三维网络结构岩体力学的核心技术。     

Groundwater flow into underground openings in fractured crystalline rocks: an interpretation based on long channels

Rethinking an old tracer experiment in fractured crystalline rock suggests a concept of groundwater flow in sparse networks of long channels that is supported by results from an innovative lattice network model. The model, HyperConv, can vary the mean length of ‘strings’ of connected bonds, and the gaps between them, using two independent probability functions. It is found that networks of long channels are able to percolate at lower values of (bond) density than networks of short channels. A general relationship between mean channel length, mean gap length and probability of percolation has been developed which incorporates the well-established result for ‘classical’ lattice network models as a special case. Using parameters appropriate to a 4-m diameter drift located 360 m below surface at Stripa Mine Underground Research Laboratory in Sweden, HyperConv is able to reproduce values of apparent positive skin, as observed in the so-called Macropermeability Experiment, but only when mean channel length exceeds 10 m. This implies that such channel systems must cross many fracture intersections without bifurcating. A general relationship in terms of flow dimension is suggested. Some initial investigations using HyperConv show that the commonly observed feature, ‘compartmentalization’, only occurs when channel density is just above the percolation threshold. Such compartments have been observed at Kamaishi Experimental Mine (Japan) implying a sparse flow network. It is suggested that compartments and skin are observable in the field, indicate sparse channel systems, and could form part of site characterization for deep nuclear waste repositories.     

数字钻孔摄像在小浪底帷幕灌浆检测孔中的应用

为了评价帷幕灌浆效果,将数字钻孔摄像技术应用到小浪底水利枢纽3号灌浆洞3个灌浆质量检查孔中。通过数字钻孔摄像得到的孔壁图像资料, 对钻孔内发育的节理裂隙、破碎带和砂浆充填情况进行了分析研究,结果表明：?灌浆时浆液在外压下,主要沿连通性较好、延伸较远的裂隙向孔外岩体渗透。与渗流路径相连通的裂隙或延伸较短的裂隙,被浆液直接充填凝固后形成水泥结石。与渗流路径没有连通的裂隙则被忽略,或在灌浆压力下劈裂而部分充填,造成全充填、半充填和砂浆流出等几种充填效果;?破碎带充填与否主要与破碎带的规模、性质和与灌浆洞连通性有关。小浪底水利枢纽3号灌浆洞内的灌浆效果是明显的;将数字钻孔摄像技术应用于检查帷幕灌浆质量,分析灌浆浆液渗流路径,也是可行的。     

Rock Type Connectivity Estimation Using Percolation Theory

Complicated sedimentary processes control the spatial distribution of geological heterogeneities. This serves to make the nature of the fluid flow in the hydrocarbon reservoirs immensely complex. Proper modeling of these heterogeneities and evaluation of their connectivity are crucial and affects all aspects of fluid flow. Since the natural variability of heterogeneity occurs in a myriad of length scales, accurate modeling of the rock type connectivity requires a very fine scheme, which is computationally very expensive. Hence, this makes other alternative methods such as the percolation approach attractive and necessary. The percolation approach considers the hypothesis that a reservoir can be split into either permeable (sand/fracture) or impermeable rocks (shale/matrix). In this approach, the connectivity of the permeable fraction governs the flow. This method links the global properties of the system to the density of the permeable objects distributed randomly in the system. Moreover, this approach reduces many results to some simple master curves from which all-possible outcomes can be predicted by simple algebraic transformations. The current study contributes to extending the applicability of the methodology to anisotropic systems as well as using the complicated and more realistic sandbody shapes (for example, ellipsoids). This enables us to attain a better assessment of the connectivity and its associated uncertainty of the complicated rock types. Furthermore, to validate the approach, the Burgan reservoir dataset of the Norouz offshore oil field in the south of Iran was used. The findings are in conformity with the percolation approach predictions.     

Fracture network versus single fractures: Measurement of fracture geometry with X-ray tomography

Several long-standing questions concerning fluid transport in fractured rock involve the geometrical properties of the connected void space within a fracture network. We present experimental results that compare the geometrical properties of a fracture network to those of the individual fractures in the network. To image and quantify the aperture distribution of natural fracture networks in coal, a Wood's metal injection method is combined with X-ray computerized tomography and image analysis. We find that the aperture distribution of the networks is spatially anisotropic and dependent on the number and geometry of the individual fractures. Void area in the individual fractures ranged between 45%–58%. A three-dimensional auto-correlation analysis on the fracture network and a two-dimensional analysis on the individual fractures found that the apertures were correlated over distances of 10 mm to 20 mm in the direction of flow.     

非饱和带裂隙岩体渗流的特点和概念模型

非饱和带裂隙岩体中的渗流和饱和带相比有其不同的特点,包括毛细管流、薄膜流、优先流和裂隙-基质相互作用等。这些特点导致非饱和带裂隙岩体中的渗流具有相当的非均质性。笔者描述了裂隙岩体中非饱和渗流的这些特点,并简单讨论了目前所存在的模拟非饱和渗流的概念模型。     

Coupled flow network and discrete element modeling of injection-induced crack propagation and coalescence in brittle rock

We present a numerical analysis on injection-induced crack propagation and coalescence in brittle rock. The DEM network coupling model in PFC is modified to capture the evolution of fracture geometry. An improved fluid flow model for fractured porous media is proposed and coupled with a bond-based DEM model to simulate the interactions among cracks induced by injecting fluid in two nearby flaws at identical injection rates. The material parameters are calibrated based on the macro-properties of Lac du Bonnet granite and KGD solution. A grain-based model, which generates larger grains from assembles of particles bonded together, is calibrated to identify the microscopic mechanical and hydraulic parameters of Lac du Bonnet granite such that the DEM model yields a ratio between the compressive and tensile strength consistent with experiments. The simulations of fluid injection reveal that the initial flaw direction plays a crucial role in crack interaction and coalescence pattern. When two initial flaws are aligned, cracks generally propagate faster. Some geometrical measures from graph theory are used to analyze the geometry and connectivity of the crack network. The results reveal that initial flaws in the same direction may lead to a well-connected crack network with higher global efficiency.

     

Alternative water resources in granitic rock: a case study from Kinmen Island,Taiwan

Kinmen Island is a small, tectonically stable, granitic island that has been suffering from a scarcity of fresh water resources due to excessive annual evapotranspiration over annual precipitation. Recent studies further indicate that shallow (0–70 m) sedimentary aquifers, the major sources of groundwater supply, have already been over-exploited. Therefore, this preliminary study is to investigate the existence of exploitable water resources that can balance the shortage of fresh water on this island. Site characterization data are obtained from island-wide geophysical surveys as well as small-scale tests performed in a study area formed by three deep (maximum depth to 560 m) vertical boreholes installed in mid-east Kinmen northeast to Taiwu Mountain. Vertical fracture frequency data indicate that the rock body is fractured with a spatially correlated pattern, from which three major fracture zones (depths 0–70, 330–360, and below 450 m) can be identified. Geologic investigations indicate that the deepest fracture zone is caused by the large-scale, steeply dipping Taiwushan fault. This fault may have caused a laterally extensive low-resistivity zone, a potential fractured aquifer, near Taiwu Mountain. The middle fracture zone is induced by the Taiwushan fault and intersects the fault approximately 21 m southeast of the study area below a depth of 350 m. Slug testing results yield fracture transmissivity varying from 4.8 × 10⁻⁷ to 2.2 × 10⁻⁴ m²/s. Cross-hole tests have confirmed that hydraulic connectivity of the deeper rock body is controlled by the Taiwushan fault and the middle fracture zone. This connectivity may extend vertically to the sedimentary aquifers through high-angle joint sets. Despite the presence of a flow barrier formed by doleritic dike at about 300 m depth, the existence of fresh as well as meteoric water in the deeper rock body manifests that certain flow paths must exist through which the deeper fractured aquifers can be connected to the upper rock body. Therefore, groundwater stored within the Taiwushan fault and the associated low-resistivity zone can be considered as additional fresh water resources for future exploitation.